Functional Morphology of the Femur in Apes and Human Beings

Akiyoshi Matsumura ¹, Takeshi Nishimura ², Yutaka Takahashi ¹, Yuzuru Hamada ², Seiichiro Inokuchi ³ and Morihiko Okada ⁴
(Department of Biology, National Defense Medical College¹,
Primate Research Institute, Kyoto University² ,
Showa University³, The University of Tsukuba⁴, )

The aim of this study is to analyze how the modes of locomotion correlate to the functions of hind limb bones in apes and human beings.
The cross-sectional geometric parameters were determined serially along the diaphysis of the femur of chimpanzees (Pan troglodytes) and humans (Homo sapiens) by using CT (computed X-ray tomographic scans). The relationships between the cross-sectional morphology of the femur and muscle attachment region of the bone were examined in the thigh of chimpanzees by using MRI (magnetic resonance imaging). The cross-sectional geometrical properties and muscle attachment region of human femora as reported by some investigators (e.g. Ruff and Hayes, 1983; Swindler and Wood, 1973) were referred to for comparison. Cross-sectional outlines and external diameters of antero-posterior (DA-P) and medio-lateral (DM-L) direction were also examined serially along the diaphysis of the femur in chimpanzees (Pan troglodytes), bonobos (Pan paniscus), australopithecus (Australopithecus afarensis; AL288-1) and humans (Homo sapiens).
In the chimpanzees, the cross-sectional area of compact bone and the area moments of inertia (Ix, Iy, Imax, Imin, Ip) increased toward the most proximal diaphysis, whereas in the humans, the area moments of inertia increased toward both the most proximal and most distal diaphysis. The cross-sectional area of compact bone in the humans marked a peak value around mid diaphysis and showed a reverse pattern for the area moments of inertia. The %CA (cortical area index; cross-sectional area of compact bone ×100 / total cross-sectional area) of chimpanzees showed higher value than humans in the proximal diaphysis. The %CA value showed a large peak around 40-50% levels in the chimpanzees and slightly distal to mid shaft in the humans. The Ix/Iy and Imin/Imax values showed a large peak around the proximal 40-45 % level in the chimpanzees and slightly distal to mid shaft in the humans. DA-P/DM-L (comparative diameter) value showed a large peak around the proximal 40-45% level in the chimpanzees and slightly distal to mid shaft in the bonobos, australopithecus and the humans.
The differences of the values among the hominoids were considered to be related to the regional differences of muscle attachments, muscle contractile forces and joint reaction forces, depending on the modes of locomotion. Bipedal locomotion may correspond to a changing pattern in the comparative area moments of inertia and comparative diameter along the femur shaft.